Exhaust gas emitted from an internal combustion engine, for example, a diesel engine, is a heterogeneous mixture that contains gaseous emissions such as carbon monoxide (CO), unburned hydrocarbons (HC) and oxides of nitrogen (NOx) as well as condensed phase materials (liquids and solids) that constitute particulate matter. Catalyst compositions typically disposed on catalyst supports or substrates are provided in an engine exhaust system to convert certain, or all of these exhaust constituents into non-regulated exhaust gas components.
In some cases, one or more selective catalytic reduction (SCR) devices are provided to reduce the amount of NOx in the exhaust. The SCR devices make use of ammonia (NH3) or other reductant to reduce the NOx. For example, when the proper amount of NH3 is available at the SCR device under the proper conditions, the NH3 reacts with the NOx in the presence of an SCR catalyst to reduce the NOx emissions to, for example, nitrogen.
Accordingly, it is desirable to provide systems and methods for controlling the amount of reductant that is available at the SCR device to reduce NOx.